Overload relays are electrical switches typically employed in industrial settings to protect electrical equipment from damage due to overheating in turn caused by excessive current flow. In a typical case, the electrical equipment is a three-phase motor which is connected to a power source through another relay commonly referred to as a contactor. A typical contactor is a heavy duty relay having three switched power paths for making and breaking each of the circuits connected to the three phase power source. The motion required to make and break the contacts is provided magnetically as a result of current flow through a coil which in turn is energized by a current whose flow is controlled by another switch, typically remotely located.
In a conventional setup, an overload relay is connected in series with the control switch for the coil of the contactor. When an overload condition is detected by the overload relay, the same cuts off power to the coil of the contactor, allowing the contactor to open and disconnect the electrical equipment that is controlled by the contactor from the source of power to prevent injury to the electrical equipment.
In the past, overload relays have utilized resistive heaters for each phase which are in heat transfer relation with a bi-metallic element which in turn controls a switch. When an overload is sensed as, for example, when there is sufficient heat input from the resistive heater to the bi-metallic element, the bi-metallic element opens its associated switch to de-energize the contactor coil and disconnect the associated piece of electrical equipment from the source of power.
More recently, the resistive heater-bi-metallic element type of relay has been supplanted by electronic overload relays. See, for example, commonly assigned U.S. Pat. No. 5,179,495 issued Jan. 12, 1993, to Zuzuly, the entire disclosure of which is herein incorporated by reference. Outputs of such circuitry typically are relatively low powered and as a consequence, in order for the output to control the contactor coil current, a solid state switch may be required. The solid state switch may, in turn, control flow to a relatively low power contact mechanism which in turn is operable to control the flow of current to the contactor as well as to operate an indicator. In the usual case, the indicator will be a light which will be illuminated upon the occurrence of a disconnect resulting from an overload. One such contact mechanism is disclosed in my commonly assigned co-pending application entitled, "Trip Mechanism for an Overload Relay", Ser. No. 08/838,904, Filed Apr. 11, 1997, the entire disclosure of which is herein incorporated by reference.
In a typical case, an overload relay, once tripped, will remain in an open position, preventing the flow of current to the contactor. Consequently, in order to resume operation of the equipment being controlled by the system, the overload relay must be reset and this is typically accomplished manually. Usually, a push button is employed so that the person operating the equipment may push the push button to cause a reset of the system, closing the contacts of the overload relay to again allow current to flow to the contactor coil which in turn will close the contacts of the contactor and provide current to the electrical equipment.
At the same time, applicable standards require that the construction of the push button and associated mechanical components of the overload relay be such that the overload relay contacts may open in the event of an overload even when the push button has been or is being pushed for reset purposes. These same standards also require that the overload relay be such that it cannot be manually defeated as, for example, by jamming the push button employed for reset in a position causing resetting of the overload relay. The purpose is to prevent damage to the electrical equipment if an overload condition occurs or continues during the process of resetting the overload relay or if the overload relay reset push button is jammed in the reset position, intentionally or otherwise. In other words, the purpose of the standard is to require that the overload relay construction be such that it cannot be defeated by holding down or jamming the push button in the reset position. An overload relay having such a feature is known as a "trip-free" overload relay.
The overload relay mechanism described in my above-identified co-pending application includes a feature whereby an indicator can be operated when an overload occurs and is a trip-free overload relay. The same works extremely well for its intended purposes, but in some instances where the push button is in the reset position and a further trip occurs while the contactor coil is energized to shut down the equipment being controlled by the system, the contacts operator for that part of the system that provides an indication of a reset or a trip may encounter the push button or associated structure before the contacts used in the indicator circuit fully close resulting in an erroneous indication of the condition of the overload relay.
The present invention is directed to overcoming one or more of the above problems.